The long-term goal of this proposal is to understand the molecular and cellular mechanisms of protein kinase C and how these mechanisms are altered by cancer-associated mutations in members of this enzyme family to promote oncogenesis. Protein kinase C isozymes transduce the myriad of signals resulting from receptor-mediated hydrolysis of phospholipids, playing critical roles in diverse cellular functions The discovery in the 1980s that they are the receptors for the potent tumor promoting phorbol esters, led to the dogma that activation of protein kinase C by phorbol esters promotes tumorigenesis. However, the finding that long-term treatment with phorbol esters causes the degradation of protein kinase C opens this interpretation to question. Protein kinase C levels are aberrant in diverse cancers, and over 160 mutations in protein kinase C isozymes have been found in human cancers. However, whether protein kinase C isozymes function as tumor suppressors or oncogenes is not established. By understanding the molecular and cellular mechanisms of protein kinase C, and taking advantage of novel tools our lab has developed, we are poised to address this fundamental question. Thus, a central hypothesis driving this proposal is that protein kinase C may be a tumor suppressor, with inactivation (rather than activation, as is the dogma) of the kinase promoting oncogenic pathways. Three Aims are proposed: 1] to advance knowledge on the molecular mechanisms of protein kinase C, 2] to understand protein kinase C signaling in cells, and 3] to examine how cancer-associated mutations in protein kinase C alter its function, testing the hypothesis that protein kinase C may be a tumor suppressor rather than oncogene.